Clean fuels research has become an important subject of environmental catalysis studies worldwide. The new government regulations in many countries all over the world including India demand the production and use of more environmentally friendly transportation fuels with lower contents of sulfur. In fact, control of noxious SOx and NOx gases from automobile engines and, for that purpose, near zero levels of S and N in transportation fuels are aimed for worldwide implementation in the coming years. Furthermore, in the meantime, the demand for transportation fuel has been increasing in most countries; especially the increase in demand for diesel fuel seems to be highly significant.
In the last decades, hydrotreating catalysts have been widely used for hydrotreating various hydrocarbon oils, more particularly for hydrodesulfurization, hydrodenitrogenation and hydrodearomatization of distillates, residual fractions from the crude oil distillation units, and hydrofining of lubricant oil fractions. Such hydrotreating catalyst should be highly active to desulfurize deeply even the weakly reactive refractory sulfur species. The chemistry of deep desulfurization of refractory sulfur compounds involves pre-hydrogenation followed by C—S bond cleavage as compared to the conventional direct desulfurization through hydrogenolysis reaction and therefore, the catalyst should be having superior design features that enable this reaction pathway for producing high quality diesel products.
These hydro treating catalysts comprise of a refractory inorganic oxide support such as alumina and metals of at least one each from Group VIB, preferably molybdenum and Group VIII, preferably cobalt or nickel of the periodic table supported on alumina. These catalysts have, on the alumina surface, layered clusters of molybdenum disulfide chemically modified with nickel or cobalt with coordinately unsaturated edge sites as active sites responsible for hydro desulfurization activity of the catalyst. Further, the basal plane of the molybdenum disulfide clusters has hydrogenation-active edge metallic sites which enable hydrogenation of refractory sulfur species and aromatics. The morphological features of the molybdenum disulfide layered metal sulfide clusters such as the degree of clustering and cluster length are the determining factors for catalytic activity needed for desulfurization performance.
However, there has been an increased demand for more active catalysts for effectively meeting the mandatory environmental restrictions enforced throughout the world to obtain better quality fuels with minimum level of pollutants like sulfur and nitrogen. In view of the depleting availability of lighter and sweeter crude oils worldwide, it is highly desirable for the catalyst to have high stability for enabling the processing of heavier and dirtier feedstocks containing significant amounts of sulfur, nitrogen and heavy metal contaminants like nickel and vanadium.
The composition and methodology of preparation of the catalyst carrier together with the design of catalyst active sites involving the control of physico-chemical characteristics, optimized interactions of active metals with the support surface and metal solution chemistry leads to enhanced activity and stability of the catalyst.